The strategy for implementing damage detection and the characterization of mechanical structures is commonly called Structural Health Monitoring (SHM). Damage is defined as modifications of the material and/or of the geometrical properties of a structural system, comprising modifications of boundary conditions and connections of the system, that worsen performance of the system. The SHM process implies the observation of the mechanical system along the time using periodically: measurements of dynamic responses coming from an array of sensors, extraction of data of damage characteristics sensed from these measurements, and statistical analysis of these data of characteristics for determining the present health state of the system (also called structural analysis).
Currently, SHM systems use sensors located outside the surfaces to be controlled. For example, in bridges a number of sensors are used (anemometers for calculating the wind speed, accelerometers, extensometers, motion transducers, temperature sensors, sensors for detecting motion of weights, etc.), all placed on the external surfaces of beams, ropes or pillars, in order to: estimate the effects of loads on the bridge, evaluate the weakening of the bridge, and foresee the probable evolution of the bridge and its expected lifetime.
SHM systems with sensors buried in the structures to be monitored have been devised. These sensors (of pressure, humidity, temperature, etc.) have at least an antenna for remote power supply and for transmitting outside the block itself the measured values. Sensors of this kind are disclosed for example in the US patent application No. 2004/0153270 and in the Italian patent applications No. VA2010A000097 and MI2010A002365, the entire contents of which are incorporated herein by reference.